add methods for mapping error responses

[robjtede]

PR Type

Feature

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

Give ability for users, middleware in particular, to alter the response generated from Errors.

[Samielakkad]

The idea of response mappers is interesting, but the public API as written has an extra generic type parameter that callers cannot infer:

pub fn add_mapper<F, B>(&mut self, mapper: F)
where
    F: Fn(HttpResponse) -> HttpResponse + 'static,

B is not used in the argument list, return type, or bounds, so a normal call like:

err.add_mapper(|res| res)

will require the caller to specify a meaningless generic parameter or fail type inference. This should probably just be pub fn add_mapper<F>(...) unless B was intended to be part of the mapper signature.

It would also be good to add a small compile/pass test or unit test that actually calls add_mapper through the public API, so this kind of inference issue is caught.

[robjtede]

@JohnTitor basically an attempt to work around actix/actix-extras#680 in a nonbreaking way